Depend what you are planning to do, and what ESC you are running. If you are building primarily for bashing around, I would recommed a 6 cell sub C "sport" pack, in the 3300 to 3800 Mah range. This will be totally safe for any ESC, and probably give over an hour of run time while crawling.
If you are building to compete, I would probably recommend a little more voltage, either 7 or 8 cells, in a 2/3 A size cell, either the IB1400, or Elite 1500 cells. Half the capacity means half the run time, but you get a little extra punch when you need it, and the pack is smaller and lighter. I run 8 cell IB1400 packs in my 2.2 comp rig from cheapbatterypacks.com.